Because of the characteristics of light weight, good mechanical property, being usable for making products with a specific shape by molding, expanded polypropylene (EPP) beads are a widely used polymer foam material, the development and industrial production thereof have been the focus of national industry and academia. Compared with the molded products made from polystyrene series resin foam beads, the expanded polypropylene molded body which is obtained by molding the expanded polypropylene beads has excellent chemical resistance, high toughness, high heat resistance, good compression resilience, etc. However, the industrial EPP has shortcomings, such as high molding temperature, poor flame retardant antistatic performance, and poor impact resistance at a low temperature.
Firstly, the energy consumption of molding process is high. When expanded polypropylene beads are molded in-mold, it is necessary to heat by using a steam with a higher saturated vapor pressure, in order to allow the foam particles, melt together while the foam particles are secondary foamed. Therefore, it is necessary to use a metal mold with high pressure resistance and a special molding machine with high stamping, and also leads to an increase in energy consumption. So it is very important to develop EPP beads molding technology with low vapor pressure and low temperature.
Secondly, EPP foam beads are flammable. Polypropylene is a flammable substance, burning with large heat release, and accompanied by molten droplets, easy to spread the flame. In addition, EPP beads have cell structure, and worse flame retardant properties. At this stage most of the EPP beads can't achieve flame retardant function, which limits its applications in the field of high flame retardant demand. At present, on the domestic market, the flame resistant PP is prepared mainly from the composite flame retardant comprising halogen-containing organic compounds and antimonous trioxide. Halogen-containing flame retardant plastic products will produce toxic, corrosive gases and a lot of smoke in the combustion, which can cause great harm to the environment. In recent years, halogen-containing flame retardant materials in many environmental assessment reports are referred to release highly toxic carcinogens, such as benzofurans and dioxins etc. during the processing, burning and recycling process thereof, which serious harm to the environment and human health. In February 2003, the European Union first announced the ROHS directive (electronic motor products hazardous substances limit instructions) to limit the halogen, all of Germany, the United States, Japan, China are also enacted the relevant environmental laws and regulations. The global producers, suppliers and customers of electrical and electronic equipment make the most insurance requirements “zero halogen” within the supply chain, in order to allow their own products and production lines to meet existing and future environmental regulations.
At present, the widely used halogen-free polypropylene flame retardants are including hydroxide, phosphorus and nitrogen and the compound thereof. The hydroxide flame retardant is represented by magnesium hydroxide and aluminum hydroxide, and the amount is more than 60 wt % to make the polypropylene reach the UL94V0 flame retardant grade requirement for the insulating sheet, but this leads to difficulties in flame retardant polypropylene processing. The phosphorus flame retardant is represented by red phosphorus and organic phosphoric acid esters, and the amount is lower than that of hydroxides, but the insulation grade of polypropylene plates is reduced due to the large water absorption rate and the high leakage rate of products. The nitrogen flame retardant is represented by melamine and triazine, but it can't make the products thereof achieve a high flame retardant grade when the thickness of molded body or plate in the range of 0.125-0.75 mm. Therefore, it is of great practical significance to develop an environmentally friendly flame retardant PP with low smoke and zero halogen.
Thirdly, EPP beads have poor antistatic properties. When EPP beads are molded as the relevant electronic materials packaging and liquid crystal panel turnover box, there is a higher requirement for the antistatic performance of EPP beads. General expanded PP material has poor antistatic property, which is easy to produce static charges when rub or peel with the outside. Moreover, the produced charges are not easy to leak out, and continue to accumulate on the surface. When polypropylene surface charged, it will absorb the dust and dirt in the air, if without effective surface or antistatic treatment. When the human body exposed to the static polypropylene, there will be a feeling of electric shock. And static electricity can also cause the malfunction of electronic equipment. More seriously, the accumulation of static electricity will result in the phenomenon of static attraction (or repulsion), electric shock or spark discharge, this will lead to a huge disaster under the environment of the flammable, explosive material. In order to avoid the influence of static electricity, the polypropylene needs to be antistatic modified to adapt some special occasions.
It is one of the main processes for preparing polymeric antistatic composites to add conductive functional components (such as conductive carbon black) or antistatic agents into the polymer matrix. However, in general, the filled amount of conductive filler or the amount of the antistatic agent required to form the conductive network are relatively large, resulting in a significant reduction in the mechanical properties of the polymer, and improving the production cost and process difficulty of the material. Therefore, reducing the amount of conductive filler is an important part of the development and application of antistatic composites. Chinese Patent Application 2005100040230 discloses a preparation of the polyolefin resin foam body with antistatic property by using polymer antistatic agent, which having a surface intrinsic resistivity of 108Ω to 1013Ω. The used polymer antistatic agent mainly comprises the block copolymer of polyether and polypropylene, the mixture of polyether ester amide and polyamide, etc., while the amount of antistatic agent is 4-6%, and it is a short-acting antistatic agent, antistatic property can only last for 30 days. Chinese Patent Application 2007101922158 discloses a process for preparing antistatic and insulating (anti-conductive) polypropylene. The volume resistivity of the obtained polypropylene particles is adjustable between 10°-1011 Ω·cm, the amount of carbon black is 25%-35%; because of the surface density of carbon black is low, the amount is large, blending with polypropylene base resin is difficult, increasing the complexity of the process and product cost.
The most important is after adding flame retardants and long-acting antistatic agent into polypropylene beads, EPP beads' the cell structure and the mechanical properties of the molded body will be significantly influenced, the quality of the molded products which is subsequent molded is difficult to be guaranteed, which limit its application areas. When both of flame retardant and antistatic agents are added, they often result in simultaneous decline of flame retardancy or antistatic properties for each other.
Fourth, the impact resistance of polypropylene at a low temperature is poor, especially propylene homopolymer. The impact polypropylene which is obtained by adding the rubber dispersed phase has excellent impact resistant strength at a high temperature or a low temperature, high tensile strength, bending modulus and other rigidity and high heat resistance temperature, which has been widely used in many fields such as molding or extruding auto parts, household appliances, containers and household items. The foam beads which are prepared by using impact polypropylene also have good resistant to low temperature, especially have a broad prospect in a cold chain transport packaging, sports equipment, building insulation, aerospace. Because of the low melt strength of the traditional general-purpose impact polypropylene, there are some problems in the preparation of foam beads, such as merging and fracturing of the cells, poor molding ability and so on.
A common process for increasing the melt strength of polypropylene is to reduce the melt index, i.e., to increase the molecular weight of polypropylene, but this could lead to difficulty in melting and extruding material. Another process is to broaden the molecular weight distribution. For example, U.S. Pat. Nos. 7,365,136 and 6,875,826 discloses respectively a process for preparing the homopolymerization and random copolymerization polypropylene with broad molecular weight distribution, high melt strength, wherein the alkoxysilane is selected as an external electron donor (such as dicyclopentyldimethoxysilane), the effect of increasing the melt strength of the polypropylene is achieved by adjusting the concentration of hydrogen to control the molecular weight and distribution in a plurality of series reactors. WO 9426794 discloses a process for preparing high melt strength homopolymerization and random polypropylene in a plurality of series reactors, which produce high melt strength polypropylene with broad molecular weight distribution or bimodal distribution by adjusting the concentration of hydrogen in different reactors, the nature of the catalyst is not adjusted in each reactor, so the process for preparing requires a large amount of hydrogen. CN 102134290 and CN 102134291 discloses respectively a process for preparing homopolypropylene with a wide molecular weight distribution and a high melt strength, which produces a homopolypropylene or random copolymer polypropylene with a wide molecular weight distribution and a high melt strength by using a plurality of series reactors through controlling the species and proportions of the external electron donor components at different reaction stages, and then combining the controlling of the amount of hydrogen which is function as the molecular weight modifier. Chinese Patent Application No. 201210422726.5 also discloses a process for preparing to obtain homopolypropylene or random copolymerized polypropylene with wide molecular weight distribution and high melt strength, which control the isotactic index and hydrogen tone sensitivity of the catalysts between the different reactors by a reasonable mixing of two different types of external electron donors of silanes and diethers.